Field of the Disclosure
The present disclosure relates to an organic light emitting diode display having a uniform light emission efficiency (or, ‘luminous efficiency’) throughout the entire pixel area. Especially, the present disclosure relates to an organic light emitting diode display having an organic light emitting layer in which the light emission efficiency of the blue light wavelength has the same level of light emission efficiency as the other light wavelengths.
Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and high volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP), and the electroluminescence device (or EL).
The electroluminescence display device is categorized into the inorganic light emitting diode display device and the organic light emitting diode display device according to the luminescence material. As a self-emitting display device, the electroluminescence display device has the merits that the response speed is fast, the brightness is high, and the view angle is wide.
FIG. 1 is a diagram illustrating the structure of an organic light emitting diode according to Related Art. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode generates light due to the energy from an excition formed at an excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode display can image video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer ELM of the organic light emitting diode as shown in FIG. 1.
The organic light emitting diode display (or OLED) using the organic light emitting diode can be categorized as a passive matrix type organic light emitting diode display (or PMOLED) or an active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows the video data by controlling the current applied to the organic light emitting diode using a thin film transistor (or TFT).
FIG. 2 is a circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED) according to Related Art. FIG. 3 is a plane view illustrating the structure of one pixel in the AMOLED according to Related Art. FIG. 4 is a cross sectional view along the cutting line I-I′ for illustrating the structure of the AMOLED also of Related Art.
Referring to FIGS. 2 and 3, the active matrix organic light emitting diode display comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLED connected to the driving thin film transistor DT. The scan line SL, the data line DL, and the driving current line VDD are formed on the substrate SUB to define the pixel area. The organic light emitting diode OLED is formed within the pixel area to define the light emitting area.
The switching thin film transistor ST is formed where the scan line SL and the data line DL is crossing. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the scan line SL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OLED disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLED. The organic light emitting layer OLE is inserted between the anode electrode ANO and the cathode electrode CAT. The cathode electrode CAT is connected to the base voltage (or, ground voltage) VSS. There is a storage capacitor Cst disposed between the gate electrode DG of the driving thin film transistor DT and the driving current line VDD or between the gate electrode DG of the driving thin film transistor DT and the drain electrode DD of the driving thin film transistor DT.
Referring to FIG. 4 in more detail, on the substrate SUB of the active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively, are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the gate contact hole GH penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
As mentioned above, the substrate SUB having the thin film transistors ST and DT has uneven surfaces and level differences because there are many elements. It is preferable for the organic light emitting layer OLE to be formed on an even surface to ensure uniform light emitting distribution over the entire area of the organic light emitting layer OLE. Therefore, in order to make the surface of the substrate SUB smooth, the over coat layer OC is deposited over the substrate SUB.
On the over coat layer OC, an anode electrode ANO of the organic light emitting diode OLED is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT via the pixel contact hole PH formed at the over coat layer OC and the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank (or ‘bank pattern’) BN is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL, and VDD, for defining the light emitting area. The exposed portion of the anode electrode ANO by the bank BN would be the light emitting area. On the anode electrode ANO exposed from the bank BN, the organic light emitting layer OLE is formed. On the organic light emitting layer OLE, the cathode electrode CAT is formed.
FIG. 4 illustrates an example of the full-color bottom emission type organic light emitting diode display. In this case, the color filter CF may be further included between the over coat layer OC and the passivation layer PAS, and the anode electrode ANO may be made of the transparent conductive material. The organic light emitting layer OLE may include an organic material emitting white light. Further, the organic light emitting layer OLE and the cathode electrode CAT may be deposited as covering the whole surface of the substrate.
In order to represent the natural full color with the organic light emitting diode display as mentioned above, the color reproduction range of the light emitted from each pixel is important. However, in the related art of the organic light emitting diode display, many developers are concerned with the manufacturing process, the driving voltage, or efficiency more than on the color reproduction range. That is, the technology for the organic light emitting diode display has been developed by accepting a lot of the degradation of the color reproduction range.
However, as the mass production technology is required at a high level, it is more important to manufacture an organic light emitting diode display having an excellent video quality. In order to enhance the video quality, it is important to ensure the uniformity of the light emitting efficiency over all pixels.